They were made only in 2004, the number of mystic convertibles was higher than any other color convertible produced in 04, and for coupes only black and silver had more produced than mystic. The seats in mystic cars matched the exterior and so did the steering wheel i think.

http://yellowmustangregistry.com/Production04Cobra.html
2004 production numbers
Mysti-Chrome Coupes 515
Mystic-Chrome Convertibles 495